This disclosure relates generally to a method of making thermosetting compositions for use in electrical circuit materials and the resulting products, and in particular to thermosetting polybutadiene and polyisoprene circuit substrate materials.
As used herein, a circuit material is an article used in the manufacture of circuits and multi-layer circuits, and includes circuit laminates, bond plies, resin coated conductive layers, and cover films. Circuit laminates, bond plies, resin coated conductive layers, and cover films in turn are formed from dielectric materials that can comprise a thermosetting or thermoplastic polymer. The dielectric material in a bond ply, resin covered conductive layer, or cover film may comprise a substantially non-flowable dielectric material, i.e., one that softens or flows during manufacture but not use of the circuit, whereas the dielectric material in a circuit laminate (e.g., a dielectric substrate) is designed to not soften or flow during manufacture or use of the circuit or multi-layer circuit. Dielectric substrate materials are further typically divided into two classes, flexible and rigid. Flexible dielectric substrate materials generally tend to be thinner and more bendable than the so-called rigid dielectric materials, which typically comprise a fibrous web or other forms of reinforcement, such as short or long fibers or fillers.
A circuit laminate as used herein refers to one or two conductive layers fixedly attached to a dielectric substrate, which is formed from a dielectric material. Patterning a conductive layer of a laminate, e.g., by etching, provides a circuit. Multi-layer circuits comprise a plurality of conductive layers, at least one of which contains a conductive wiring pattern. Typically, multi-layer circuits are formed by laminating one or more circuits together using bond plies, and, in some cases, resin coated conductive layers, in proper alignment using heat and/or pressure. The bond plies are used to provide adhesion between circuits and/or between a circuit and a conductive layer, or between two conductive layers. In place of a conductive layer bonded to a circuit with a bond ply, the multi-layer circuit may include a resin coated conductive layer bonded directly to the outer layer of a circuit. In such multi-layer structures, after lamination, known hole forming and plating technologies may be used to produce useful electrical pathways between conductive layers.
Polybutadiene and polyisoprene thermosetting materials have been successfully employed as rigid electrical circuit substrates. These materials have typically used halogenated, particularly brominated, flame retardant additives to achieve the necessary levels of flame retardancy. In recent years, brominated flame retardants have come under scrutiny, such that certain of them will be banned by January 2008. The remaining brominated flame retardants will require special incineration/disposal procedures. In light of the impending ban, manufacturers are placing additional pressures upon suppliers to produce flame retardant additives that are effective, yet that do not contain halogens.
The most commonly used alternative flame retardant additives are phosphorous/nitrogen compounds. However, phosphorous/nitrogen compounds possess high dielectric constants, loss factors, and moisture absorption properties. These properties are adverse to intended uses in applications such as the electronic industries, automobile industries, and particularly in circuit boards and related applications. Accordingly, there remains a need for non-halogen containing flame retardant thermosetting compositions that provide the desired flame retardant properties without impairing physical properties such as electrical and moisture absorption properties.